In addition to being protective, energy absorbing and stiff, the cage also has to be light. An overbuilt cage weights the car down and makes it slow. With these tall and seemingly oxymoronic requirements, it’s easy to see that a good cage requires a combination of good fabrication techniques, engineering and common sense. A bad cage can really bring down a good car and make it harder, perhaps impossible to set up well.
Like all race car projects before you weld or cut anything you must first check the rules. We perused the NASA CCR as well as Super Streets time attack rulebook to make sure that we built something that exceeded the requirements. For materials we decided on 1020 DOM mild steel. Although mild steel does not sound as exotic as the higher strength chrome moly steel, it has several advantages for a roll cage. Chrome moly is steel with a high percentage of chromium and molybdenum. These elements impart superior mechanical properties including a 40% higher tensile strength than mild steel. Unfortunately chrome moly usually must be post weld heat treated to return this same level of strength to the heat affected zone of the welds. The metal tends to be more brittle in the zones around the weld. Chrome moly is also less ductile and tends to crack when its limits are exceeded. Mild steel however is less affected by the heat of welding and is more malleable.
|An overhead view shows additional upward struts to help support the main roof halo in a severe roll over, another trick borrowed from WRC cars.|
What this equates to is a cage that can deform, bend and absorb energy better in a serious crash while staying together and not cracking apart. You may hear contrary opinions by other highly qualified fabricators but we feel that for us, mild steel is safer. Many race sanctioning bodies feel the same way and sometimes chrome moly is against the rules, another good reason to check the rulebook first before starting on a project.
|An odd design feature is that Nissan did not box the forward frame rails completely. The sturdy upper part of the frame rail just floats in space. We speculate that this was done to control crumple in a crash. We welded a gusset here to completely box the frame rail to the firewall to maximize stiffness.|
In steel terms DOM is another important trait. DOM means drawn over mandrel. When DOM tubing is made, a flat sheet of steel is drawn into a tubular shape while cold over a mandrel and electrically seam welded to make a complete tube. This cold drawing makes the grain of the metal finer and gives it better mechanical properties. A cheaper form of mild steel tubing is called ERW which means electrical resistance welded. ERW tubing is hot rolled into tubular form and resistance welded. It does not get the grain refinement that cold worked tubes get and thus has worse mechanical properties. NASA does not allow ERW tubing for roll cages, another good reason to read the rules first!
The rules state that a car of our weight must have a minimum of 1.5” diameter main tubes with a wall thickness of 0.120” or a 1.75” tube with a wall thickness of 0.095”. We went to the larger diameter thinner walled tube as a larger diameter has a larger section modulus, in engineering terms this means a big diameter, thin walled tube is stiffer for the same amount of material than a smaller diameter think walled tube. Richie went to work bending up welding the main part of the cage. He took a lot of care to ensure that the mitering of the tubes was very precise, that the tubes were as close to the shell of the car as possible and things were done in a sequence where he could get a full 360 degree weld around every juncture, a step where many cage builders unfortunately fall short. Richie even cut small holes in the floor so the cage could be lowered, a difficult series of welds competed, then raised back up with the hole being covered by the tubes foot mounting plate. This is seriously sano.